Categories of Fires
Fires are made up of four basic types. Type A fires are made up of organic solids such as trash, wood or paper. Type B fires are made up of inflammable liquids such as kerosene, paint thinner or gasoline. Type C fires are caused by electricity that is still flowing to everything that is on fire. Type D fires are made up of burning metals such as magnesium or sodium that are reactive and will burn at high temperatures. When burning, these metals can chemically extract oxygen and carbon dioxide from water, making the fire more difficult to contain.
Pressurized extinguishers use either water or carbon dioxide that has been pressurized. Type A fire extinguishers are normally silver and use pressurized water. This type of extinguisher works by removing heat from the fire and cooling it down by saturating everything in water. These extinguishers work best with Type A fires that are made up of organic materials.
Carbon dioxide extinguishers use pressurized carbon dioxide. They work by removing the oxygen from the fire, thus removing the heat. They blanket the burning area with a layer of heavy gas and it no longer can support combustion. They are used to extinguish Type C fires that are caused by electricity or Type B fires. These extinguishers are red with large nozzles.
Chemical Compound Extinguishers
Halon extinguishers use bromochlorodifluoromethane, which is a heavy gas. They not only displace the oxygen in the fire but the chemicals in the extinguisher shut down the combustion of the fire. These extinguishers are good for Type A and C fires. Halon extinguishers are slowly being phased out due to the chlorofluorocarbons damaging the ozone layer.
Type BC extinguishers dry-chemical extinguishers that use sodium or potassium carbonate. These extinguishers work well with both Type B and C fires, but should not be used for fires that are Type A or D. These extinguishers are usually red with a small nozzle.
ABC dry-chemical extinguishers are made up of ammonium phosphate which isolates burning materials from the oxygen. Since Class D metals can free oxygen from phosphate ions, this extinguisher shouldn't be used on fires that are Type D.
Fixed Extinguishing Systems
Fixed extinguishing systems are built directly into many buildings to protect them from fire. The most common type of these systems are water sprinklers. They are not only the most reliable extinguishing system when dealing with large buildings, but they are also safe for people. These systems might not be effective in situations where space is limited or water damage will not be tolerated. These would be situations such as in an airplane or in a computer room. In these situations another type of fire extinguishing system would be put into effect that uses something other than water.
It's imperative to use the correct extinguisher with the correct fire. Carbon dioxide used in large concentrations required to put out a fire, is toxic to people and should not be used when humans are present. By using the incorrect extinguisher on the wrong type of fire, you can change chemical compounds that are burning and make the fire worse. It's important to assess the situation before choosing an extinguisher to use.